The present invention relates to large-scale fabrication of small, two-color balls, approximately 12 .mu.m in diameter, for use in an "electric paper" display sheet.
Typical electric paper displays are disclosed in U.S. Pat. Nos. 4,126,854 and 4,143,103, the subject matters of which are fully incorporated herein by reference. In general, such displays include an elastomeric host layer a few millimeters thick which is heavily loaded with hemispherically bichromal (i.e., two-color) balls. Each bichromal ball has hemispheres of contrasting colors, such as a white half and a black half. Upon application of an electrical field between electrodes located on opposite surfaces of the host layer, the balls rotate to present one or the other hemisphere to an observer, depending on the polarity of the field. The resolution of the electric paper is dependent upon the number and size of the bichromal balls loaded into the host layer. More specifically, loading a greater number of bichromal balls having smaller diameters (e.g., .apprxeq.12 .mu.m) into the host layer produces an electric paper having a higher resolution. Therefore, it is desirable to produce large numbers of bichromal balls having such smaller diameters.
Heretofore, a typical method of creating bichromal balls has included the spinning disc method which is disclosed by Crowley et al. in U.S. Pat. No. 5,262,098, the subject matter of which is fully incorporated herein by reference.
Briefly, the spinning disc method includes introducing black and white pigmented, hardenable liquids to upper and lower surfaces, respectively, of a disc mounted on a rotatable spindle. The liquids are moved to the periphery of the disc by centrifugal force where they flow together, without mixing, to form bichromal "globs." The centrifugal force causes the bichromal globs to "break-away" from the disc, during a process referred to as "break-up."
Ideally, the globs break-away from the disc in the form of small, individual spherical balls which are substantially identical and have proper bichromal characteristics (i.e., one hemisphere contains the black pigment while the other hemisphere contains the white pigment). Although the spinning disc method is capable of producing a large number of bichromal balls in a relatively short period of time, a large percent of the balls produced are unacceptable. In other words, the balls are not substantially identical to each other (e.g., the diameter of one ball may be .apprxeq.12 .mu.m while the diameter of another ball may be 80 .mu.m or greater) and/or they do not have proper bichromal characteristics.
Bichromal balls are typically produced from various polymers (e.g., waxes or other resins), having pigment loadings of 25% to 50% by weight (or less than 12% by volume), heated to temperatures of approximately 500.degree. C. to 600.degree. C. The polymer/pigment combination is referred to as a slurry. The resulting viscosity of the molten slurry at these pigment loadings and temperatures typically ranges between about 15 centipoise and about 20 centipoise. At these viscosities, however, only about 10% of the slurry input to a spinning disc production system is output as bichromal balls having acceptable characteristics. In other words, approximately 90% of the bichromal balls produced by current methods are unacceptable.
One reason for the low yield of usable balls is "ligament snap-back." Ligament snap-back is a phenomenon which results when the balls break-away from the disc too slowly. Globs which should be dispensed from the disc are instead pulled-back in the axial direction by surface tension. These globs which have been pulled-back combine with one or more subsequent globs, thereby forming a single oversized glob and, consequently, an oversized ball. Oversized balls are frequently non-spherical and have improper bichromal surface characteristics. One way to prevent ligament snap-back is to decrease the viscosity of the polymer used to form the balls, thereby preventing the slurry from breaking-away from the disc too slowly. In this manner, the forces exerted by the viscosity of the slurry become insignificant relative to the forces exerted by surface tension.
Paraffin wax, which has a viscosity between about 5 centipoise and about 6 centipoise (i.e., lower than polymers previously used for creating the slurry), has been used for preventing ligament snap-back. However, paraffin wax has a relatively lower melting point than other polymers. Also, because of the lower viscosity, the pigments suspended within the paraffin wax tend to mix between the hemispheres during the formation of the balls. Therefore, the balls formed using paraffin wax also lack preferred bichromal characteristics.
Viscous forces are typically described in terms of "viscous length". Viscous length is defined as: ##EQU1## where .eta. represents the viscosity, .gamma. represents the surface tension, and .rho. represents the mass density of a fluid. Most fluids have densities around 10.sup.3 kgM.sup.-3, and surface tensions of about 0.03 NM.sup.-1. These two properties are remarkably similar among many fluids. Therefore, a comparison of L.sub..eta. among various fluids is primarily a comparison of their viscosities.
Proper break-up is obtained when the respective diameters L.sub.D of the balls which break-away from the disc are much larger than L.sub..eta.. Since L.sub.D represents the diameter of a ball, it is also a measure of the distance between the centers of two sequential balls which have similar diameters. A somewhat quantitative measure of the quality of break-up can be obtained by comparing the characteristic time for break-up, .tau..sub.b, with the characteristic time for snap-back, .tau..sub.c. The ratio of these times then indicates the degree of competition between the forces for break-up and the forces for snap-back. It can be shown that: ##EQU2##
Proper break-up has been shown to occur when: ##EQU3## To satisfy Equation 3, the characteristic time for break-up must be significantly shorter than the characteristic time for snap-back. In other words, one glob must break-away from the disc before enough relative inertia is imparted to subsequent balls.
For slurries including low pigment loading, such as those currently used for producing twisting balls, the viscous length, L.sub..eta., is approximately 60 .mu.m. As stated above, balls currently produced by the spinning disc method have diameters, L.sub.D, of approximately 80 .mu.m. It can be seen from Equation 2 that these values yield a ratio of break-up time to snap-back time, .tau..sub.b /.tau..sub.c, of approximately 0.866. Equation 3 indicates such conditions are not ideal for proper break-up conditions to occur. Therefore, it is evident proper break-up conditions do not occur when bichromal balls are fabricated from the slurries currently used.
To obtain even finer resolutions, it is desirable to produce bichromal balls having even smaller diameters (i.e., smaller values of L.sub.D). However, Equation 2 shows that bichromal balls having a smaller diameter result in a relatively larger value for the ratio .tau..sub.b /.tau..sub.c, if the viscous length is held constant. Larger values of .tau..sub.b /.tau..sub.c indicate a longer break-up time relative to the snap-back time. Such a result, as shown by Equation 3, is undesirable and will result in even fewer acceptable balls. Furthermore, bichromal balls having smaller diameters will require a higher percent of pigment loading in the polymer to obtain proper opacity. The increased pigment loading tends to increase the viscosity of the slurry and, hence, L.sub..eta., thus resulting in even higher values for the .tau..sub.b /.tau..sub.c ratio.
The present invention provides a new and improved apparatus and method for producing large numbers of substantially spherical bichromal balls, having smaller diameters (i.e., .apprxeq.12 .mu.m) and proper bichromal characteristics, which overcome the above-referenced problems and others.